Electric cable with corrugated sheath and semi-conductive protective layer between the sheath and the core

ABSTRACT

An electric cable having one or more insulated cores with an external semi-conductive screen surrounded by a corrugated metal sheath and having a tube of soft, semi-conductive material intermediate the core and the sheath with the corrugations of the sheath embedded therein to both prevent slippage of the core with respect to the sheath and conductively interconnect the sheath and the screen. The tube may be a wound tape of semi-conductive crepe paper or such a tape surrounded by glass cloth having intercalated metal wires.

United States Patent 91 Priaroggia et al.

[451 Oct. 22, 1974 1 ELECTRIC CABLE WITH CORRUGATED SIIEATII ANDSEMI-CONDUCTIVE PROTECTIVE LAYER BETWEEN THE SI-IEATII AND THE CORE [75]Inventors: Paulo Gazzana Priaroggia, Milan;

Gabriele Maschio, Monza, both of Italy [73] Assignee: Industrie PirelliSocieta Per Azioni,

Milan, Italy [22] Filed: Oct. 5, 1973 21 Appl. No.: 403,962

[30] Foreign Application Priority Data Nov. 13, 1972 Italy 31581/72 [52]US. Cl. 174/102 D, 174/102 SC, 174/120 SC [5 l] Int. Cl. I-IOlb 7/20[58] Field of Search 174/102 D, 102 R, 102 SC,

174/105 SC, 120 SC, 121 SC, 25 R [56] References Cited UNITED STATESPATENTS 3,719,769 3/1973 Miyauchi et al. 174/105 SC 3,766,309 10/1973Calzolari 1. 174/102 D 3,773,965 11/1973 Reynolds 174/25 G PrimaryExaminer-A. J. Grimley Attorney, Agent, or FirmBro0ks, Haidt & Haffner[57] ABSTRACT 6 Claims, 2 Drawing Figures l SCREENS C/ (45 F re g zCORRUGATED 2 a 17 l is 111% 1 The present invention relates toimprovements in electric cables provided with a metal sheath, inparticular, a corrugated aluminum sheath, and of the type described inco-pending application Ser. No. 162,199, filed July 13, 1971, andentitled ELECTRIC CABLE WITH CORRUGATED METALLIC SHEATH, now U.S. Pat.No. 3,766,309. The invention is applicable to high voltage power cablesand especially to Millikentype cables.

Said U.S. Pat. No. 3,766,309 describes an electric cable comprising acore, i.e., a conductor surrounded throughout its length by at least onelayer of insulating material, which is surrounded by a corrugated metalsheath. A soft tubular covering, generally of the order of about one ormore millimeters in radial dimension, covers said core, the corrugationsof the sheath, at least for a part of their depth, penetrating into saidsoft tubular covering in order to provide a co-penetration of the latterand the sheath. A preferred embodiment of the invention illustrated insaid patent is an electric cable comprising a core contained in acorrugated metal sheath, in which said soft tubular covering for thecore is made of crepe paper.

In an electric cable of the above-described type, the possible slippageof the core with respect to the sheath, when the cable is installed invertical direction, is substantially prevented. Moreover, the softtubular covering, in particular when it is made of crepe paper, hasprovided, in practice, very suitable for the dissipation of heat in thecase of power cables subjected to thermal cycles, thereby avoidingundesired over-heating of the dielectric.

However, in certain high voltage cables provided with a corrugatedsheath, for example, the cables known to those skilled in the art asMilliken-type cables, due to the lack of an electrical contact betweenthe outermost screen of the core and the corrugated sheath, capacitivecurrents may occur which, flowing in the screen, could give rise toconsiderable power losses and to a subsequent over-heating of amagnitude which cannot be balanced by the soft tubular covering. Also,abnormal voltages may be produced between the screen and the metalsheath, which could be the cause of a breakdown discharge.

The principal object of the invention of the present application is toprovide an electric cable having a corrugated metal sheath which isimproved with respect to the electric cable described in said patent,the improvement being also applicable to high voltage power cables, inparticular, to Milliken-type cables, and the improvement substantiallyeliminating the hereinforementioned disadvantages.

More precisely, the object of the present invention is an improvedelectric cable comprising a core contained in a corrugated metal sheath,and of the type disclosed in said application, in which a soft tubularcovering surrounds said core, the corrugations of the sheath, at leastfor a part of their depth, penetrating in said soft tubular covering soas to provide a co-penetration between the latter and the sheath, saidcable being characterized in that said soft tubular covering isconstituted by a material having a high electric conductivity withrespect to a dielectric material.

In the preferred embodiment of the invention, the electric cable, saidsoft tubular covering, made of a material having a high electricconductivity, is constituted by a winding of tapes of semi-conductivecrepe paperfl-w- In another embodiment of the invention, said soft tuvbular covering, made of a material having a high electric conductivity,is constituted by an internal winding of tapes of semi-conductive crepepaper and by an external winding of a cloth of glass fibers containingintercalated metal wires.

The objects and advantages of the invention will be apparent to thoseskilled in the art from the following description of the presentlypreferred embodiments thereof, which description should be considered inconjunction with the accompanying drawing in which:

FIG. 1 represents, in longitudinal section, a single core, power cablehaving a core surrounded by a corrugated metal sheath and having anintermediate layer of semi-conductive crepe paper; and

FIG. 2 represents, in longitudinal section, a Millikentype cable,provided with a corrugated aluminum sheath and a composite intermediatelayer of semiconductive crepe paper and a cloth of glass fiberscontaining intercalated metal wires.

The cable represented in longitudinal section in FIG. 1 is a single corecable and comprises a core 10 and a corrugated sheath 1]. Although thesheath 11 illustrated in FIG. 1 has helical corrugations, thecorrugations could be annular. The core 10 comprises a strandedconductor 12, on which is wound a first, or internal, semi-conductivescreen 13 surrounded by a dielectric layer 14, which, in turn, issurrounded by a second, or external, semi-conductive screen 15. The core10 is electrically complete, that is, except for the need to protect itfrom physical damage with a sheath 1 l, the core 10 may be used forcarrying current between separated points.

The core 10 is protected from damage by the sheath 11, both during theapplication of the sheath 11 and during subsequent use and is preventedfrom slipping with respect to the sheath 1 1 by a soft tubular coveringor layer 16, made of a material having a high electric conductivity ascompared to ordinary insulation, e.g., it has a resistivity, MD, of from30 to 5,000 ohm cm. and a bulk resistivity of from 3,000 to 500,000 ohmcm. In the embodiment illustrated, the covering 16 is constituted by ahelical winding of tapes of semiconductive crepe paper thereby forming atube of solid, self-sustaining and resilient material between the sheath1] and the core 10. As described in said patent, the material of thecovering 16 is different from the material of the layer 14 and is softerthan the latter so as to permit deformation of the sheath 11 withoutsubstantial deformation of the layer 14. The covering 16 frictionallyengages the external screen 15 and the sheath 11, at least partiallyfilling the corrugations of the sheath 11, and thereby preventssignificant relative movement between the sheath 1 l and the core 10when the cable is suspended by the sheath 11. The shear strength of thecovering 16 is such that it can prevent such relative movement withoutshearing of the covering 16 or the material thereof. The normal exteriorsize of the covering 16 in the absence of deformation thereof is greaterthan the minimum diameter of the corrugations of the sheath 11.

The inner surface 16 of the soft, tubular, conductive covering 16 is incontact with the external semiconductive screen 15, and the outersurface 18 of said covering 16 is in contact with the internal surfaceof the corrugated sheath 11, at least along a helical surface in theform of a generally cylindrical helix of circular section,

coincident with the inner, smallest diameter face 19 of thecorrugations. The contact between the soft, tubular, conductive covering16 and the sheath l1, besides ensuring the hereinbefore describedadvantages of transmitting to the core the anchoring forces applied tothe sheath 1 1 and of permitting a very good dissipation of heat sothat, at an equal rated current, it is possible to have cables ofreduced section, namely,

\ lighter than those previously used, provides a conductive electricalinterconnection between the external, semi-conductive screen and thesheath l1, preventing the existence of capacitive currents andconsequently, the risk of undue over-heating and breakdown discharges.

The cable illustrated in FIG. 2 is of the type known to those skilled inthe art as a MIlliken-type cable. In said cable, the core 20 issurrounded by a corrugated metal sheath 21 having annular corrugations,but the corrugations could be helical as illustrated in FIG. 1.

The conductor of the core 20 comprises three conductive strands 22 inthe form of sectors which are helically would together about thelongitudinal axis aa of the cable and defining a generally cylindricalpassageway 23 which constitutes the duct for the supply of the cableimpregnating fluid which may, for example, be sulfur hexa-fluoride,nitrogen, a mixture of sulfur hexafluoride and nitrogen, fluid oil, etc.

About the helically wound strands 22, there is a winding of a metal tape24, which maintains the sectors 22 in their cylindrical helixconfiguration, so that the cross-section of the conductor of the core 20is kept as nearly circular as possible. On the metal tape 24, there is afirst, intemal, semi-conductive screen 25 surrounded by a dielectriclayer 26 which, in turn, is surrounded by a second, external,semi-conductive screen 27. The latter is protected by a soft,conductive, tubular covering 28 similar to, and for the same purposesas, the covering 16. v

In the embodiment shown in FIG. 2, the soft, tubular covering 28comprises an internal winding 29 of semiconductive crepe paper and anexternal winding 30 of a known type of cloth made of glass fibers andcontaining intercalated metal wires, namely, good electricallyconductive elements which, extending from one part of the cloth to theother, can conductively interconnect What is claimed is:

1. An electric cable comprising at least one core including a conductorsurrounded throughout substantially its length by at least one layer ofinsulating material and an external screening layer of at leastsemiconductive material surrounding said insulating layer,

a corrugated metal sheath surrounding said core and a tube comprisingsolid, at leat semi-conductive material intermediate said core and saidsheath and in frictional engagement and electrical contact with saidscreening layer and said sheath and at least partially filling the twocontiguous layers between which the cloth of glass fibers containingintercalated metal wires is interposed. In the embodiment shown, aconductive connection is established between the internal winding 29 ofsemiconductive crepe paper of the soft, tubular covering 28 and thecorrugated sheath 21.

The cloth of glass fibers 30 containing intercalated metal wires hasalso the advantages of acting as a thermal and mechanical protector ofthe core 20 in the excorrugations of said sheath to prevent slipping ofsaid core with respect to said sheath and to conductively interconnectsaid screening layer and said sheath, said tube beings self-sustaining,resilient and softer than the material of said core to permitdefomiation of said tube by the corrugations of said sheath withoutsubstantial deformation of said core and said tube having a shearstrength sufficient to prevent said slippage without shearing when saidcable is suspended by said sheath.

2. An electric cable comprising at least one core including a conductorsurrounded throughout its length by at least one layer of insulatingmaterial and an external screening layer of at least semi-conductivematerial surrounding said insulating layer, a corrugated metal sheathsurrounding said core, and a tube of solid, at least semi-conductive,material intermediate said core and said sheath with the corrugations ofsaid sheath embedded in said tube for preventing slipping of said coreand said sheath relative to each other in a direction extendinglongitudinally of said cable when said cable is suspended by said sheathand for conductively interconnecting said screening layer and saidsheath, said tube comprising at least one tape made of an insulatingmaterial different from that of said insulating layer, deformable by thecorrugations of said sheath and having a normal exterior size in theabsence of deformation thereof by said corrugations which is greaterthan the minimum diameter of said corrugations so that portions of saidtube at least partially fill said corrugations and said tube encirclingsaid core and having at least a portion of its interior in frictionalengagement with the exterior of said core thereby to provide a tube ofsolid material between said core and said sheath in frictionalengagement with both said sheath and said core which substantiallyprevents significant relative movement between said core and said sheathlongitudinal of said cable when said cable is suspended by said sheath,said material of said tape being softer than the material of said coreso as to permit deformation and compression of said tube by saidcorrugations of said sheath without substantial deformation of said coreand said material of said tape also being self-sustaining and resilientand having a shear strength sufficient to prevent said relative movementwithout shearing thereof.

3. An electric cable comprising at least one core including a conductorsurrounded throughout substantially its length by at least one layer ofinsulating material and an external screening layer of at leastsemiconductive material surrounding said insulating layer, a corrugatedmetal sheath surrounding said core and a tube of semi-conductive tape ofcrepe paper wound around said screening layer and intermediate said coreand said sheath and in frictional engagement and electrical contact withsaid screening layer and said sheath and at least partially filling thecorrugations of said sheath to prevent slipping of said core withrespect to said sheath and to conductively interconnect said screeninglayer and said sheath, said tube being selfsustaining, resilient andsofter than the material of said core to permit deformation of said tubeby the corrugations of said sheath without substantial deformation ofsaid core and said tube having a shear strength sufficient to preventsaid slippage without shearing when said cable is suspended by saidsheath 4. An electric cable comprising at least one core including aconductor surrounded throughout substantially its length by at least onelayer of insulating material and an external screening layer of at leastsemiconductive material surrounding said insulating layer, a corrugatedmetal sheath surrounding said core and a tube of semi-conductive tape ofcrepe paper wound around said screening layer and a further winding ofglass fiber cloth containing intercalated metal wires wound around saidtape and intermediate said core and said sheath and in frictionalengagement and electrical contact with said screening layer and saidsheath and at least partially filling the corrugations of said sheath toprevent slipping of said core with respect to said sheath and toconductively interconnect said screening layer and said sheath, saidtube being self-sustaining, resilient and softer than the material ofsaid core to permit deformation of said tube by the corrugations of saidsheath without substantial deformation of said core and said tube havinga shear strength sufficient to prevent said slippage without shearingwhen said cable is suspended by said sheath.

5. An electric cable comprising at least one core including a conductorsurrounded throughout its length by at least one layer of insulatingmaterial and an external screening layer of at least semi-conductivematerial surrounding said insulating layer, a corrugated metal sheathsurrounding said core, and a tube of solid, at least semi-conductive,material intermediate said core and said sheath with the corrugations ofsaid sheath embedded in said tube for preventing slipping of said coreand sid said tube for preventing slipping of said core and said sheathrelative to each other in a direction extending longitudinally of saidcable when said cable is suspended by said sheath and for conductivelyinterconnecting said screening layer and said sheath, said tubecomprising at least one tape made of an insulating material differentfrom that of said insulating layer and being made of semi-conductivecrepe paper and said tube being deformable by the corrugations of saidsheath and having a normal exterior size in the absence of deformationthereof by said corrugations which is greater than the minimum diameterof said corrugations so that portions of said tube at least partiallyfill said corrugations and said tube encircling said core and having atleast a portion of its interior in frictional engagement with theexterior of said core thereby to provide a tube of solid materialbetween said core and said sheath in frictional engagement with bothsaid sheath and said core which substantially prevents significantrelative movement between said core and said sheath longitudinally ofsaid cable when said cable is suspended by said sheath, said material ofsaid tape being softer than the material of said core so as to permitdeformation and compression of said tube by said corrugations of saidsheath without substantial deformation of said core and said material ofsaid tape also being self-sustaining and resilient and having a shearstrength sufficient to prevent said relative movement without shearingthereof.

6. An electric cable as set forth in claim 5, wherein said tubecomprises a winding of glass fiber cloth containing intercalated metalwires would around said tape.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECHQN Patent No. 3 843830 Dated October 22 1974 Inventor) PAOLO GAZZANA PRIAROGGIA andGABRIELE MASCHIO It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Col. 4 (claim 1) line 25 "beings" should read -being I Col. 6 (claim 5)line 9 is a combination of lines 8 and 10 and should be deleted Signedand sealed this 1st day of July 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officerand Trademarks UNITED STATES PATENT OFFICE 3 CERTIFICATE OF CORRECTIONPatent No. 3,843,830 'Dated October 22, 1974 Inventor s PAOLO GAZZANAPRIAROGGIA and GABRIELE MASCHIO It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

Col. 4 (claim l) line 25 "beings" should read -b eing- Col. 6 (claim 5)line 9 is a combination of lines 8 and 10 and should be deleted.

Signed and sealed this 1st day of July 1975.

(SEAL) Attest:

C. I-IARSHALL DANN RUTH C. MASON Commissioner of Patents ArrestingOfficer and Trademarks

1. An electric cable comprising at least one core including a conductorsurrounded throughout substantially its length by at least One layer ofinsulating material and an external screening layer of at leastsemi-conductive material surrounding said insulating layer, a corrugatedmetal sheath surrounding said core and a tube comprising solid, at leatsemi-conductive material intermediate said core and said sheath and infrictional engagement and electrical contact with said screening layerand said sheath and at least partially filling the corrugations of saidsheath to prevent slipping of said core with respect to said sheath andto conductively interconnect said screening layer and said sheath, saidtube beings self-sustaining, resilient and softer than the material ofsaid core to permit deformation of said tube by the corrugations of saidsheath without substantial deformation of said core and said tube havinga shear strength sufficient to prevent said slippage without shearingwhen said cable is suspended by said sheath.
 2. An electric cablecomprising at least one core including a conductor surrounded throughoutits length by at least one layer of insulating material and an externalscreening layer of at least semi-conductive material surrounding saidinsulating layer, a corrugated metal sheath surrounding said core, and atube of solid, at least semi-conductive, material intermediate said coreand said sheath with the corrugations of said sheath embedded in saidtube for preventing slipping of said core and said sheath relative toeach other in a direction extending longitudinally of said cable whensaid cable is suspended by said sheath and for conductivelyinterconnecting said screening layer and said sheath, said tubecomprising at least one tape made of an insulating material differentfrom that of said insulating layer, deformable by the corrugations ofsaid sheath and having a normal exterior size in the absence ofdeformation thereof by said corrugations which is greater than theminimum diameter of said corrugations so that portions of said tube atleast partially fill said corrugations and said tube encircling saidcore and having at least a portion of its interior in frictionalengagement with the exterior of said core thereby to provide a tube ofsolid material between said core and said sheath in frictionalengagement with both said sheath and said core which substantiallyprevents significant relative movement between said core and said sheathlongitudinal of said cable when said cable is suspended by said sheath,said material of said tape being softer than the material of said coreso as to permit deformation and compression of said tube by saidcorrugations of said sheath without substantial deformation of said coreand said material of said tape also being self-sustaining and resilientand having a shear strength sufficient to prevent said relative movementwithout shearing thereof.
 3. An electric cable comprising at least onecore including a conductor surrounded throughout substantially itslength by at least one layer of insulating material and an externalscreening layer of at least semi-conductive material surrounding saidinsulating layer, a corrugated metal sheath surrounding said core and atube of semi-conductive tape of crepe paper wound around said screeninglayer and intermediate said core and said sheath and in frictionalengagement and electrical contact with said screening layer and saidsheath and at least partially filling the corrugations of said sheath toprevent slipping of said core with respect to said sheath and toconductively interconnect said screening layer and said sheath, saidtube being self-sustaining, resilient and softer than the material ofsaid core to permit deformation of said tube by the corrugations of saidsheath without substantial deformation of said core and said tube havinga shear strength sufficient to prevent said slippage without shearingwhen said cable is suspended by said sheath
 4. An electric cablecomprising at least one core including a conductor surrounded throughoutsubstantially its length by at least one layer of insulating materialand an external screening layer of at least semi-conductive materialsurrounding said insulating layer, a corrugated metal sheath surroundingsaid core and a tube of semi-conductive tape of crepe paper wound aroundsaid screening layer and a further winding of glass fiber clothcontaining intercalated metal wires wound around said tape andintermediate said core and said sheath and in frictional engagement andelectrical contact with said screening layer and said sheath and atleast partially filling the corrugations of said sheath to preventslipping of said core with respect to said sheath and to conductivelyinterconnect said screening layer and said sheath, said tube beingself-sustaining, resilient and softer than the material of said core topermit deformation of said tube by the corrugations of said sheathwithout substantial deformation of said core and said tube having ashear strength sufficient to prevent said slippage without shearing whensaid cable is suspended by said sheath.
 5. An electric cable comprisingat least one core including a conductor surrounded throughout its lengthby at least one layer of insulating material and an external screeninglayer of at least semi-conductive material surrounding said insulatinglayer, a corrugated metal sheath surrounding said core, and a tube ofsolid, at least semi-conductive, material intermediate said core andsaid sheath with the corrugations of said sheath embedded in said tubefor preventing slipping of said core and sid said tube for preventingslipping of said core and said sheath relative to each other in adirection extending longitudinally of said cable when said cable issuspended by said sheath and for conductively interconnecting saidscreening layer and said sheath, said tube comprising at least one tapemade of an insulating material different from that of said insulatinglayer and being made of semi-conductive crepe paper and said tube beingdeformable by the corrugations of said sheath and having a normalexterior size in the absence of deformation thereof by said corrugationswhich is greater than the minimum diameter of said corrugations so thatportions of said tube at least partially fill said corrugations and saidtube encircling said core and having at least a portion of its interiorin frictional engagement with the exterior of said core thereby toprovide a tube of solid material between said core and said sheath infrictional engagement with both said sheath and said core whichsubstantially prevents significant relative movement between said coreand said sheath longitudinally of said cable when said cable issuspended by said sheath, said material of said tape being softer thanthe material of said core so as to permit deformation and compression ofsaid tube by said corrugations of said sheath without substantialdeformation of said core and said material of said tape also beingself-sustaining and resilient and having a shear strength sufficient toprevent said relative movement without shearing thereof.
 6. An electriccable as set forth in claim 5, wherein said tube comprises a winding ofglass fiber cloth containing intercalated metal wires would around saidtape.